PHOENIX, 20 Mar 2015 (AFP) - Ko, at 17 the youngest golfer ever to claim the world number one ranking, shook off the weather delay that pushed back the start of the round by four hours.
Teeing off on 10, she had three birdies in her first nine holes. She added birdies at the first, third and fifth before her only miscue of the day, a three-putt bogey at the seventh.
She rebounded with her seventh birdie of the day at the par-four eighth and capped her round with a par to share the clubhouse lead with Germany's Sophia Popov and Americans Kim Kaufman and Tiffany Joh.
Ko said her score could have been lower.
"I think I had like three (putts) that were perfectly on line but I left it short, and that three-putt on seven," she said. "But I guess everything balances out.
"I hit the pin with my chip in 18 -- that could have been a much further par putt. So I think six-under is a good start and I just have got to take the positives out of it."
Darkness halted play with half of the 132-strong field yet to finish.
After waking up before 5:00 am only to see her tee time pushed back, Ko said she was looking forward to a good night's sleep.
But she said the rain-softened course had its advantages.
"They weren't really easy," she said of the conditions, "but even if we were hitting a long club into the greens, because it was soft, it was more receptive.
"I guess you might lose some distance with the drives but then you can be a bit more aggressive on the greens."
Ko has now broken par in 21 straight LPGA tour rounds, a streak that started at last year's season-ending Tour Championship.
The South Korean-born Kiwi, who owns six LPGA Tour titles and 10 worldwide, arrived in Phoenix after a strong showing overseas.
She won the Women's Australian Open and the New Zealand Women's Open in back-to-back weeks before finishing runner-up to former world number one Park In-Bee of South Korea in the HSBC Women's Champions in Singapore earlier this month.
Popov and Joh both posted rounds with six birdies and no bogeys, while Kaufman had seven birdies with one bogey to join the leading group.
They were one stroke in front of South Korea's Chun In-Gee and Thailand's Moriya Jutanugarn, both in the clubhouse on 67, with Dewi Claire Schreefel of the Netherlands also at five-under through 10 holes.
Leading partial first-round scores on Thursday in the LPGA Founders Cup (round halted by darkness, to resume on Friday. USA unless noted, par-72):
66 - Kim Kaufman, Sophia Popov (GER), Lydia Ko (NZL), Tiffany Joh
67 - Moriya Jutanugarn (THA), Chun In-Gee (KOR)
68 - Feng Simin (CHN), Jang Ha-Na (KOR), Kim Soo-Bin (KOR), Pornanong Phatlum (THA), Amy Yang (KOR), Gerina Piller, Austin Ernst